1. Field of the Invention
The present invention relates to a control device for an induction motor, and more particularly to a control device for coping with a variation in the load by changing the rotation speed according to a variation in the torque of the load.
2. Description of the Related Art
The conventional induction motor uses a VVVF (variable voltage and variable frequency) type voltage inverter in which both of the voltage and frequency can be varied. When the torque of the load varies, the rotation speed is changed. In this case, since the magnetic flux in the motor varies in proportion to the voltage/frequency ratio, a voltage is lowered to keep the ratio V/f of the output voltage v to the frequency f at a constant value when the frequency is lowered to lower the rotation speed of the motor. The V/f ratio represents a frequency-voltage characteristic of the motor, the conventional inverter contains a plurality of V/f characteristic patterns, and the user selects one of the V/f characteristic patterns according to the type of the load, motor and the circumstance by trial-and-error.
However, it is difficult to determine a V/f pattern which is suitable for a system in which the load varies, and in practice, an excessively large V/f ratio must be selected and it is not satisfactory from the viewpoint of energy saving. That is, since the V/f ratio is not changed according to a variation in the motor load, the slippage s of the motor becomes excessively small when the load is small, and in contrast, the slippage s becomes excessively large when the load is large, thereby lowering the motor efficiency.
Thus, in the conventional induction motor controlling device, since the voltage V is not controlled according to the load torque, the slippage is deviated from a desired value by a variation in the load torque, thereby lowering the efficiency.